Resin molding that uses a mold resin has been dominant in the past as a packaging method of semiconductor devices. In recent years, however, the application of wafer level package capable of reducing not only a package cost but also a package size has been expanded. In wafer level packaging, a semiconductor wafer and a cap wafer are bonded by various kinds of methods. The bonding methods can be broadly classified into a direct bonding method that directly bonds the wafers to each other and an indirect bonding method that bonds the wafers by using bonding members. Anodic bonding and normal temperature bonding are known as the direct bonding method. Fusion bonding using a solder material or low melting point glass as bonding members or adhesion bonding using an adhesive resin has been used as the indirect bonding method.
However, both of the bonding methods need a heating step. Generally, when such heating is applied, a substrate having a semiconductor wafer and a cap wafer bonded thereto (hereinafter called “bonded substrate”) undergoes large warpage owing to the difference of coefficient of thermal expansion between the semiconductor wafer and the cap wafer. This warpage of the bonded substrate may result in inferior performance of a product and product handling defect.
Various bonding processes have been proposed as means for reducing warpage of the bonded substrates described above.
Patent Literature 1, for example, describes a wafer bonding process for reducing warpage of bonded substrates by executing wafer bonding of semiconductor wafers (or cap wafers) to which half cut is applied in advance.
Patent Literature 2 describes a wafer bonding process for reducing warpage of bonded substrates which process utilizes an exothermic reaction of reactive foil, bonds wafers by local heating for a short time and reduces warpage of the bonded substrates.